There are various industrial uses for cylindrical pressure vessels such as high-pressure pump cylinders and gun barrels. These tubular components are typically subjected to high internal pressure and may be exposed to pressure fluctuations, thermal shock, and a corrosive environment which may all lead to crack initiation and fatigue crack growth on the inner diameter of the component. For example, in a typical cannon gun barrel, the operating conditions may result in a large array of radial cracks developing from the inner surface of the barrel. To increase the maximum allowable pressure in the cannon barrel as well as to reduce its susceptibility to cracking, desired residual stresses may be introduced to the inner diameter of the tubular component, for example, by an autofrettage process.
Autofrettage is a metal fabrication technique used on tubular components to provide increased strength and fatigue life to the tube by creating a compressive residual stress at the bore. During the autofrettage process, a pressure is typically applied within a component resulting in the material at the inner surface undergoing plastic deformation while the material at the outer surface undergoes elastic deformation. The result is that after the pressure is removed, there is a distribution of residual stress, providing a residual compressive stress on the inner surface of the component.
The autofrettage process may be created in a number of ways including explosive, hydraulic or mechanical means. For example, hydraulic autofrettage typically uses high hydrostatic pressure that is applied inside the tube. The tube may be sealed at both ends, the inner diameter filled with fluid, and a pressure applied to the fluid. The pressure applied to the inner diameter of the tube is high enough to plastically deform the bore of the tube but not high enough that it plastically deforms the outer diameter and bursts the tube apart. In mechanical autofrettage, a tube having an inner diameter slightly less than its desired final dimension typically has a slightly oversized die or mandrel pushed through its bore. The dimensions of the initial inner diameter and the mandrel are calculated to strain the material past its elastic limit into plastic deformation so that the final strained diameter is the final desired bore dimension. Once the mandrel is removed, the elastic recovery of the outer portion of the tube puts the now permanently deformed inner portion into compression, providing a residual compressive stress. The magnitude of this residual stress is highly dependent on the amount of material yielding that is induced during this process, which is in turn governed by geometric tolerances and material properties. The compressive residual stresses at the inner diameter of the component induced by the autofrettage process reduce the probability of crack initiation and slow down the growth rate of fatigue cracks, thus prolonging the fatigue life of the tubular component.
These autofrettage processes, however, have some limitations. For example, the stress applied along various parts of the inner diameter are typically uniform and cannot be varied along the length of the tube. In addition, potentially harmful conditions may arise when processing the component. For example, in hydraulic autofrettage, the high pressurized fluids and high pressure seals that are used may fail causing a rapid and uncontrolled release of high-pressure fluid. In mechanical autofrettage, the high pressure equipment used to force the mandrel through the tube or the equipment holding the tube in place may fail causing the rapid and uncontrolled release of the tube and/or mandrel.